Why you can read this headline

Published 4:00 am, Tuesday, August 10, 1999

PALO ALTO - Physicists from the Bay Area and around the world are competing to answer a question that sounds more religious than scientific: Why do we exist?

You, this newspaper, the room around you, the planet Earth and the whole cosmos have at least one thing in common: They're all made of matter.

Yet the simplest widely accepted picture of the universe, known as the Standard Model of physics, hints that matter shouldn't exist. Rather, the cosmos should be a dark void. That's because all matter should have been destroyed billions of years ago by colliding with its physical counterpart, antimatter.

Now, scientists from Stanford, the Lawrence Berkeley National Laboratory and other institutions are racing to be first to discover conclusive evidence of a theory that could explain why matter exists - because nature contains a built-in bias or asymmetry that favors the production of slightly more matter than antimatter.

According to the theory, this bias guaranteed that a slight excess of matter would survive the matter-antimatter annihilation after the Big Bang that spawned our universe billions of years ago. That tiny excess includes you and the cosmos around you, from pussycats to planets and gators to galaxies.

"This is a critical issue in particle physics. . . . (If the theory is correct), you and I wouldn't be sitting here if it weren't for this tiny asymmetry," physicist Jonathan Dorfan of Stanford Linear Accelerator Center near Stanford University said Monday on the opening day of a campus physics conference, the International Symposium on Lepton and Photon Interactions.

The conference, one of the last big physics confabs of the century, has attracted hundreds of physicists from around the world. It runs through Saturday.

A detector called Babar

To test the theory, SLAC scientists have been since May generating subatomic particles and their antimatter variants, known respectively as B-mesons and anti-B-mesons.

They do so using the so-called "B factory," a $177 million particle accelerator with a $110 million particle detector called "Babar," partly in honor of the comic-book elephant.

Initial results are likely by mid-2000. Teams from around the world are racing to be first to discover clear evidence of this asymmetry, specifically as manifested in subtle variations in the rates at which B-mesons and anti-B-mesons decay - convert into new particles.

In the race, SLAC physicists hope to beat their traditional rivals at another big accelerator lab in Illinois, plus labs in Japan, Germany and elsewhere.

Yet as shown by Monday's meeting - full of smiling faces and handshakes around several coffee pots on a green lawn outside the meeting room - it's a friendly competition, with teams freely sharing information on their findings and techniques.

Partly because of its cosmological implications, "this experiment has excited me like none of my previous experiments, and I have been fortunate to work on extraordinarily good (experiments) before," says Dorfan. On Sept. 1 he is scheduled to replace Burton Richter as director of SLAC, one of the world's centers of research on high-energy physics.

Physicists first discovered antimatter in the 1930s in the form of cosmic ray particles dropping from space. Decades later, theorists speculated that the universe had begun with a kind of explosion dubbed the Big Bang, which (they argued) should have generated equal numbers of matter and antimatter particles.

Classic symmetry

This argument reflected a key tenet of classical physics: The universe is symmetric, in the sense that particles and forces would behave the same way relative to each other in a mirror image universe as they do in our universe. A crude example is a game of billiards reflected in a mirror: The billiard balls' motions relative to each other are the same whether viewed directly or in the mirror.

There was one problem, though: When matter and antimatter collide, they annihilate into pure energy. Hence in the early universe, equal amounts of matter and antimatter should have utterly extinguished both, leaving nothing - a pitch-black void, with nary an atom of matter left.

So why does matter exist? Conceivably giant clumps of antimatter might exist elsewhere in the universe. But if so, these clumps should have leaked antimatter particles that we could - but haven't - detected in Earth's upper atmosphere.

Physicists suspect that the answer lies in a mysterious phenomenon called "CP violation," according to which this aspect of nature is not symmetric - rather, it is slightly prejudiced in favor of matter. In other words, nature is asymmetric, not symmetric. ( "CP" stands for charge-parity.)

That nature contains subtle asymmetries, detectable only with sensitive instruments, has been known since the 1950s.

For example, in the early 1960s physicists James Cronin, Val Fitch and others found a minuscule difference in the rates at which particles called K-mesons decayed compared with their antimatter counterparts (anti-K-mesons).

Smoking gun

The real smoking gun evidence of a built-in cosmic prejudice against antimatter, though, should come from experiments at SLAC and elsewhere on B-mesons and anti-B-mesons.

B-mesons are much heavier than, and hence can yield far more subatomic components than, K-mesons (just as a big pinata, when battered by children, spills more goodies than a little pinata).

Physicists need lots and lots of those subatomic components to determine statistically how B-mesons behave and whether they are skewed in ways that would produce more matter than antimatter on the cosmic scale.

At the conference Monday, Dorfan and other physicists reported initial results from their experiments. For example, Manfred Paulini of Lawrence Berkeley National Laboratory described how he and his colleagues at the Tevatron accelerator at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., had generated a few hundred B-meson and anti-B-meson events, then analyzed their decay rates.

Sure enough, the decay rates appear to vary slightly between the two types of particles. But the number of events recorded at Batavia is still too small - by about three-fourths - to be statistically significant, Fermilab director Michael Witherell cautioned in an interview.&lt;